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UNIT 8—LESSON 2

USE OF CONSTANT-PRESSURE CHARTS

OVERVIEW 

Identify the uses for the 1000-, 850-, 700-, 500-, 300-, 200-, 150-, 100-, 50-, and 25-mb constant-pressure charts.

OUTLINE

The 1000-mb chart

The 850-mb chart

The 700-mb chart

The 500-mb chart

The 300-mb chart

The 200-mb chart

The 150-, 100-, 50-, and 25-mb charts

CONSTANT-PRESSURE CHARTS

Upper-air charts are termed CONSTANT-PRESSURE CHARTS, because they depict conditions at levels (heights) within the atmosphere where the pressure is the same (constant).

Constant-pressure charts are produced for standard levels. These levels are the 1,000-, 850-, 700-, 500-, 400-, 300-, 250-, 200-, 150-, 100-,70-, 50-, 30-, 20-, 10-, 7-, 5-, 3-, 2-, and 1-mb levels. However, the most commonly produced charts are for the 1000-, 850-, 700-, 500-, 300-, 200-, and 100-mb levels.

Atmospheric soundings show that pressure changes most rapidly when the temperatures are cold and least rapidly when they are warm. Remember, pressure is a function of the weight of the atmosphere, and the atmosphere’s weight is dependent on its density. Cold air is more dense than warm air; therefore cold air is heavier and exerts more pressure at a given altitude. Assuming that two columns of air (one cold, one warm) exert the same pressure at the surface, the column containing the warm air has to extend to a greater height. Figure 8-2-1 illustrates the pressure-height differences in cold and warm air. Also, note the vertical spacing (thickness) between the constant-pressure levels.


Figure 8-2-1.—Pressure-height differences in cold and warm air. 

As the temperatures in the atmosphere change, so do the heights of the constant-pressure levels. Based on the U.S. standard atmosphere, the approximate heights of the more common constant-pressure levels are as follows:


Constant-pressure charts are primarily used as an aid in weather forecasting. When they are used in conjunction with surface synoptic charts, the following determinations may be made:

1. Movements of weather systems

2. Areas of cyclonic and anticyclonic windflow

3. Types of air masses

4. Location of moist and dry areas within the atmosphere

5. Formation, intensification, and dissipation of pressure systems

6. Actual slopes of fronts

7. Vertical extent of pressure systems

8. Location and strength of jet streams

Learning Objective: Identify the uses of constant-pressure charts commonly pre-pared and displayed in most weather offices.

The following is a brief summary of the principal uses of constant-pressure charts commonly prepared and displayed in most weather offices.

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